1. Field of the Invention
This invention relates generally to battery pack cell balancing and, more particularly, to a method and system for balancing the state of charge of cells in a battery pack which uses both feedback and feedforward controls, and which takes into account state of charge differences caused by both self-discharge rate deviations between cells and capacity deviations between cells, as well as deviations of parasitic load of electronic measurement/control system.
2. Discussion of the Related Art
Electric vehicles and gasoline-electric or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid electric vehicles offer several desirable features, such as reducing or eliminating emissions and petroleum-based fuel consumption at the consumer level, and potentially lower operating costs. A key component/subsystem of electric and hybrid electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle's cost. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life are key considerations in the design and operation of electric and hybrid electric vehicles.
In a battery pack consisting of hundreds of cells, there is inevitably some variation in the performance of individual cells, and this variation increases as a battery pack ages. For example, if all cells in a battery pack are fully charged and the battery pack is then used to power a vehicle, some cells will discharge slightly faster than others, due to internal resistance and self-discharge rate deviations. Energy storage capacity may also vary from cell to cell. In order to maximize battery pack durability and driving range of the vehicle, it is important to minimize the variations in state of charge from cell to cell. A common method of balancing state of charge between cells in a battery pack is to selectively discharge higher state of charge cells through a resistor. The amount of resistive discharging of each cell is typically determined as a function of the cell's state of charge relative to the average state of charge of all cells in the battery pack.
While the current cell balancing method described above is somewhat effective in balancing state of charge between cells in a battery pack, it does not take cell capacity deviation into account. That is, a cell with a lower energy storage capacity may have a higher state of charge at the beginning of a driving cycle because that cell charged at a faster rate than other cells in the battery pack. If it is a lower capacity that causes a cell to have a higher state of charge at the beginning of a driving cycle, that cell may naturally balance state of charge with the rest of the battery pack during the driving cycle, and resistively discharging that cell may over-compensate and cause the cell to discharge too quickly.
There is a need for a battery pack cell state of charge balancing method which takes into account both self-discharge rate and energy storage capacity deviations between cells. Such a method could improve cell state of charge balancing performance and minimize unnecessary resistive discharging along with the associated battery pack aging, thus leading to increased customer satisfaction through improved battery pack life and increased vehicle driving range.